Darwin's theory was mainly based on:

Explanation

Charles Darwin's theory of evolution is primarily based on the concept of natural selection. Natural selection is a process where organisms with traits that are better suited to their environment have higher chances of survival and reproduction. Over time, these advantageous traits become more common within the population, leading to evolutionary changes. The other options, such as mutation and migration, are important in evolutionary biology, but they are not the primary mechanisms that Darwin proposed. Mutations introduce genetic variation, and migration can redistribute it among populations, but natural selection is the key driver of adaptive evolution according to Darwin.